Stacked Photovoltaic Device And Method Of Manufacturing The Same

ABSTRACT

A back metal electrode, a bottom cell using microcrystalline silicon for a photoelectric conversion layer, a front cell using amorphous silicon for a photoelectric conversion layer, and a transparent front electrode are formed in this order on a supporting substrate. At least one of the concentration of impurities contained in the front photoelectric conversion layer and the concentration of impurities contained in the bottom photoelectric conversion layer is controlled such that the concentration of impurities in the bottom photoelectric conversion layer is higher than the concentration of impurities in the front photoelectric conversion layer. Impurities do not include a p-type dopant or an n-type dopant but are any one, two, or all of carbon, nitrogen, and oxygen.

This application is a Continuation of U.S. patent application Ser. No.11/307,956, filed Feb. 28, 2006, which Claims priority to JapanesePatent Application No. 2005-054963, filed Feb. 28, 2005, both of whichare incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a stacked photovoltaic device having aplurality of photovoltaic cells each comprising a photoelectricconversion layer composed of a non-single crystalline semiconductorstacked therein and a method of manufacturing the same.

2. Description of the Background Art

In recent years, photovoltaic cells using thin film-based semiconductorssuch as amorphous silicon for photoelectric conversion layers have beendeveloped. Amorphous silicon has features of abounding in raw materials,being low in manufacturing energy and manufacturing cost, making a widevariety of supporting substrates usable, making high voltagesextractable, and easily increasing in area. On the contrary, thephotovoltaic cells using amorphous silicon (hereinafter referred to asamorphous-based photovoltaic cells) are more greatly light-degraded, sothat they have lower photoelectric conversion efficiencies, as comparedwith crystal-based photovoltaic cells.

On the other hand, photovoltaic cells using microcrystalline silicon forphotoelectric conversion layers have been developed. The photovoltaiccells using microcrystalline silicon (hereinafter referred to asmicrocrystal-based photovoltaic cells) have photoelectric conversionefficiencies that are less reduced by light degradation and can absorblight in wider wavelength ranges, as compared with amorphous-basedphotovoltaic cells. In the microcrystal-based photovoltaic cells,therefore, photoelectric conversion efficiencies can be improved.

Stacked photovoltaic devices having amorphous-based photovoltaic cellsand microcrystal-based photovoltaic cells stacked therein (tandem typephotovoltaic devices or hybrid solar cells) have been developed (see JP11-243218 A, for example). In the stacked photovoltaic devices,amorphous-based photovoltaic cells are arranged on the side of lightincidence, microcrystal-based photovoltaic cells are arranged below theamorphous-based photovoltaic cells, and the amorphous-based photovoltaiccells and the microcrystal-based photovoltaic cells are connected inseries. Such stacked photovoltaic devices can receive optical spectra inwide regions, so that photoelectric conversion efficiencies areimproved. Consequently, the stacked photovoltaic devices are promisingas high efficiency thin film solar cells for power use.

In the stacked photovoltaic devices, however, the amorphous-basedphotovoltaic cells are more greatly light-degraded by light irradiation,as compared with the microcrystal-based photovoltaic cells. Therefore,balances between the output characteristics of the amorphous-basedphotovoltaic cells and the output characteristics of themicrocrystal-based photovoltaic cells are disrupted, so that the outputcharacteristics of the whole stacked photovoltaic devices aredeteriorated. As a result, the total power generations are low asobserved in the long term.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a stacked photovoltaicdevice having output characteristics after light degradation whosereduction is restrained and a method of manufacturing the same.

An intrinsic semiconductor in the specification is a semiconductor inwhich an n-type dopant or a p-type dopant is not intentionally doped,and covers a semiconductor including an n-type dopant or a p-type dopantinherently included in a raw material for the semiconductor or an n-typedopant or a p-type dopant naturally contained in the manufacturingprocess.

In the following description, impurities refer to impurities other thanelements serving as a donor or an acceptor.

A stacked photovoltaic device having a light incidence surface accordingto an aspect of the present invention comprises a plurality ofphotovoltaic cells stacked and each including a photoelectric conversionlayer composed of a substantially intrinsic semiconductor, thephotoelectric conversion layer in the one photovoltaic cell closest tothe light incidence surface including an amorphous semiconductor, thephotoelectric conversion layer in another photovoltaic cell including anon-single crystalline semiconductor containing crystal grains, and theconcentration of impurities contained in the photoelectric conversionlayer in the other photovoltaic cell is higher than the concentration ofimpurities contained in the photoelectric conversion layer in the onephotovoltaic cell.

In the stacked photovoltaic device, the photoelectric conversion layerin the one photovoltaic cell closest to the light incidence surfaceincludes the amorphous semiconductor, and the photoelectric conversionlayer in the other photovoltaic cell includes the non-single crystallinesemiconductor containing the crystal grains. Since an optical spectrumin a wide region can be received, therefore, the photoelectricconversion efficiency is improved.

The concentration of impurities contained in the photoelectricconversion layer in the other photovoltaic cells is higher than theconcentration of impurities contained in the photoelectric conversionlayer in the one photovoltaic cell closest to the light incidencesurface. Thus, light degradation of the one photovoltaic cell and lightdegradation of the other photovoltaic cell by light irradiation arebalanced. As a result, deterioration of the output characteristics ofthe whole stacked photovoltaic device by long-term use is restrained, sothat the long-term power generation thereof is improved.

The non-single crystalline semiconductor may be a microcrystallinesemiconductor containing crystal grains having a diameter of not morethan 1 μm. In this case, the photoelectric conversion layer in thesecond or subsequent photovoltaic cell from the side of the lightincidence surface comprise the microcrystalline semiconductor, so thatthe light degradation thereof is little. Consequently, the lightdegradation of the whole stacked photovoltaic cell by light irradiationis sufficiently restrained.

It is preferable that the impurities include carbon, and theconcentration of carbon contained in the photoelectric conversion layerin the other photovoltaic cell is higher than the concentration ofcarbon contained in the photoelectric conversion layer in the onephotovoltaic cell. Thus, the light degradation of the one photovoltaiccell and the light degradation of the other photovoltaic cell by lightirradiation are balanced.

It is preferable that the impurities include nitrogen, and theconcentration of nitrogen contained in the photoelectric conversionlayer in the other photovoltaic cell is higher than the concentration ofnitrogen contained in the photoelectric conversion layer in the onephotovoltaic cell. Thus, the light degradation of the one photovoltaiccell and the light degradation of the other photovoltaic cell by lightirradiation are balanced.

It is preferable that the impurities include oxygen, and theconcentration of oxygen contained in the photoelectric conversion layerin the other photovoltaic cell is higher than the concentration ofoxygen contained in the photoelectric conversion layer in the onephotovoltaic cell. Thus, the light degradation of the one photovoltaiccell and the light degradation of the other photovoltaic cell by lightirradiation are balanced.

A method of manufacturing a stacked photovoltaic device according toanother aspect of the present invention, comprising the step of forminga plurality of photovoltaic cells each comprising a photoelectricconversion layer composed of a substantially intrinsic semiconductor inorder, the photoelectric conversion layer in the one photovoltaic cellclosest to a light incidence surface including an amorphoussemiconductor, and the photoelectric conversion layer in anotherphotovoltaic cell including a non-single crystalline semiconductorcontaining crystal grains; and adjusting at least one of the formationcondition of the photoelectric conversion layer in the one photovoltaiccell and the formation condition of the photoelectric conversion layerin the other photovoltaic cell such that the concentration of impuritiescontained in the photoelectric conversion layer in the otherphotovoltaic cell is higher than the concentration of impuritiescontained in the photoelectric conversion layer in the one photovoltaiccell.

According to the method of manufacturing the stacked photovoltaicdevice, the photoelectric conversion layer in the one photovoltaic cellclosest to the light incidence surface comprises the amorphoussemiconductor, and the photoelectric conversion layer in the otherphotovoltaic cell comprises the non-single crystalline semiconductorcontaining crystal grains. Since an optical spectrum in a wide regioncan be received, therefore, the photoelectric conversion efficiency isimproved.

The concentration of impurities contained in the photoelectricconversion layer in the other photovoltaic cells is higher than theconcentration of impurities contained in the photoelectric conversionlayer in the one photovoltaic cell closest to the light incidencesurface. Thus, the light degradation of the one photovoltaic cell andthe light degradation of the other photovoltaic cell by lightirradiation are balanced. As a result, deterioration of the outputcharacteristics of the whole stacked photovoltaic device by long-termuse is restrained, so that the long-term power generation is improved.

The non-single crystalline semiconductor may be a microcrystallinesemiconductor containing crystal grains having a diameter of not morethan 1 μm. In this case, the photoelectric conversion layer in thesecond or subsequent photovoltaic cell from the side of the lightincidence surface comprises the microcrystalline semiconductor, so thatthe light degradation thereof is little. Consequently, the lightdegradation of the whole stacked photovoltaic cell by light irradiationis sufficiently restrained.

The impurities may include carbon, and the adjusting step may comprisethe step of adjusting at least one of the formation condition of thephotoelectric conversion layer in the one photovoltaic cell and theformation condition of the photoelectric conversion layer in the otherphotovoltaic cell such that the concentration of carbon contained in thephotoelectric conversion layer in the other photovoltaic cell is higherthan the concentration of carbon contained in the photoelectricconversion layer in the one photovoltaic cell.

Thus, the light degradation of the one photovoltaic cell and the lightdegradation of the other photovoltaic cell by light irradiation arebalanced.

The impurities may include nitrogen, and the adjusting step may comprisethe step of adjusting at least one of the formation condition of thephotoelectric conversion layer in the one photovoltaic cell and theformation condition of the photoelectric conversion layer in the otherphotovoltaic cell such that the concentration of nitrogen contained inthe photoelectric conversion layer in the other photovoltaic cell ishigher than the concentration of nitrogen contained in the photoelectricconversion layer in the one photovoltaic cell.

Thus, the light degradation of the one photovoltaic cell and the lightdegradation of the other photovoltaic cell by light irradiation arebalanced.

The impurities may include oxygen, and the adjusting step may comprisethe step of adjusting at least one of the formation condition of thephotoelectric conversion layer in the one photovoltaic cell and theformation condition of the photoelectric conversion layer in the otherphotovoltaic cell such that the concentration of oxygen contained in thephotoelectric conversion layer in the other photovoltaic cell is higherthan the concentration of oxygen contained in the photoelectricconversion layer in the one photovoltaic cell.

Thus, the light degradation of the one photovoltaic cell and the lightdegradation of the other photovoltaic cell by light irradiation arebalanced.

Other features, elements, characteristics, and advantages of the presentinvention will become more apparent from the following description ofpreferred embodiments of the present invention with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing the configuration of astacked photovoltaic device according to an embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment

FIG. 1 is a schematic sectional view showing the configuration of astacked photovoltaic device according to an embodiment of the presentinvention.

As shown in FIG. 1, a back metal electrode 3, a microcrystal-basedphotovoltaic cell (hereinafter referred to as a bottom cell) 200 usingmicrocrystalline silicon for a photoelectric conversion layer (a powergeneration layer), an amorphous-based photovoltaic cell (hereinafterreferred to as a front cell) 300 using amorphous silicon for aphotoelectric conversion layer, and a transparent front electrode 10 areformed in this order on a supporting substrate 100. A collection gridelectrode 11 is formed on the transparent front electrode 10.

The supporting substrate 100 has a stacked structure of a stainlessplate 1 and a polyimide resin layer 2. The back metal electrode 3 iscomposed of Au (gold), Ag (silver), Al (aluminum), Cu (copper), Ti(titanium), W (tungsten), Ni (nickel), etc. In the present embodiment,Ag is used for the back metal electrode 3.

The bottom cell 200 comprises an n-layer 4 composed of an n-typehydrogenated microcrystalline silicon film (n-type μc-Si:H), a bottomphotoelectric conversion layer 5 composed of an intrinsic (i-type)hydrogenated microcrystalline silicon film (i-type μc-Si:H), and ap-layer 6 composed of a p-type hydrogenated microcrystalline siliconfilm (p-type μc-Si:H) in this order.

The front cell 300 comprises an n-layer 7 composed of an n-typehydrogenated microcrystalline silicon film (n-type μc-Si:H), a frontphotoelectric conversion layer 8 composed of an intrinsic (i-type)hydrogenated amorphous silicon film (i-type a-Si:H), and a p-layer 9composed of a p-type hydrogenated amorphous silicon carbide film (p-typea-SiC:H) in this order.

The transparent front electrode 10 is composed of a metal oxide such asITO (indium tin oxide), SnO₂ (tin oxide), or ZnO (zinc oxide). In thestacked photovoltaic device shown in FIG. 1, the transparent frontelectrode 10 serves as a light receiving surface. In the presentembodiment, ITO is used for the transparent front electrode 10.

The back metal electrode 3 and the transparent front electrode 10 areformed by a sputtering method, for example. The bottom cell 200 and thefront cell 300 are formed by a plasma CVD (Chemical Vapor Deposition)method, for example.

In the present embodiment, at least one of the concentration ofimpurities contained in the front photoelectric conversion layer 8 andthe concentration of impurities contained in the bottom photoelectricconversion layer 5 are controlled such that the concentration ofimpurities in the bottom photoelectric conversion layer 5 is higher thanthe concentration of impurities in the front photoelectric conversionlayer 8. Here, impurities are any one, two, or all of carbon, nitrogen,and oxygen.

At least one of the concentration of carbon contained as impurities inthe front photoelectric conversion layer 8 and the concentration ofcarbon contained as impurities in the bottom photoelectric conversionlayer 5 are controlled such that the concentration of carbon in thebottom photoelectric conversion layer 5 is higher than the concentrationof carbon in the front photoelectric conversion layer 8. Alternatively,at least one of the concentration of nitrogen contained as impurities inthe front photoelectric conversion layer 8 and the concentration ofnitrogen contained as impurities in the bottom photoelectric conversionlayer 5 are controlled such that the concentration of nitrogen in thebottom photoelectric conversion layer 5 is higher than the concentrationof nitrogen in the front photoelectric conversion layer 8.Alternatively, at least one of the concentration of oxygen contained asimpurities in the front photoelectric conversion layer 8 and theconcentration of oxygen contained as impurities in the bottomphotoelectric conversion layer 5 are controlled such that theconcentration of oxygen in the bottom photoelectric conversion layer 5is higher than the concentration of oxygen in the front photoelectricconversion layer 8.

The concentration of impurities in the front photoelectric conversionlayer 8 can be controlled by adjusting reaction conditions at the timeof formation of the front photoelectric conversion layer 8. For example,the concentration of impurities in the front photoelectric conversionlayer 8 can be reduced by reducing reaction pressure at the time offormation of the front photoelectric conversion layer 8. Theconcentration of impurities in the front photoelectric conversion layer8 can be reduced by reducing the amount of H₂ (a hydrogen gas) withwhich a raw material gas is diluted at the time of formation of thefront photoelectric conversion layer 8.

The concentration of impurities in the bottom photoelectric conversionlayer 5 can be controlled by adjusting reaction conditions at the timeof formation of the bottom photoelectric conversion layer 5. Forexample, the concentration of impurities in the bottom photoelectricconversion layer 5 can be increased by increasing reaction pressure atthe time of formation of the bottom photoelectric conversion layer 5.The concentration of impurities in the bottom photoelectric conversionlayer 5 can be increased by increasing the amount of H₂ (a hydrogen gas)with which a raw material gas is diluted at the time of formation of thebottom photoelectric conversion layer 5.

In a case where the reaction pressure at the time of formation of thefront photoelectric conversion layer 8 is low or a case where the amountof the hydrogen gas serving as a diluent gas is small, or a case wherethe high-frequency power is small, the deposition rate is reduced.Consequently, the density of silicon atoms mainly composing the frontphotoelectric conversion layer 8 is increased. As a result, impuritiescontained in very small amounts in the raw material gas are difficult toincorporate in the front photoelectric conversion layer 8, so that it isconsidered that the concentration of impurities in the frontphotovoltaic conversion layer 8 is reduced.

In a case where the reaction pressure at the time of formation of thebottom photoelectric conversion layer 5 is high, the deposition rate isincreased. Consequently, the density of silicon atoms mainly composingthe bottom photoelectric conversion layer 5 is reduced. As a result,impurities contained in very small amounts in the raw material gas aredifficult to incorporate in the bottom photoelectric conversion layer 5,so that it is considered that the concentration of impurities in thebottom photovoltaic conversion layer 5 is increased.

In a case where the reaction pressure at the time of formation of thefront photoelectric conversion layer 8 is low or a case where the amountof the hydrogen gas serving as a diluent gas is small, hydrogen radicalshaving a high concentration are prevented from turning out impuritiesadhering to an electrode of a CVD system or a wall of a reactionchamber. As a result, impurities are difficult to incorporate in thefront photoelectric conversion layer 8, so that it is considered thatthe concentration of impurities in the front photovoltaic conversionlayer 8 is reduced.

On the other hand, in a case where the reaction pressure at the time offormation of the bottom photoelectric conversion layer 5 is high or acase where the amount of the hydrogen gas serving as a diluent gas islarge, hydrogen radicals having a high concentration are prevented fromturning out impurities adhering to an electrode of a CVD system or awall of a reaction chamber. As a result, impurities are easy toincorporate in the bottom photoelectric conversion layer 5, so that itis considered that the concentration of impurities in the bottomphotovoltaic conversion layer 5 is increased.

The respective concentrations of impurities in the bottom photoelectricconversion layer 5 and the front photoelectric conversion layer 8 can bealso controlled such that the concentration of impurities in the bottomphotoelectric conversion layer 5 is higher than the concentration ofimpurities in the front photoelectric conversion layer 8 by introducinga gas containing carbon, nitrogen, or oxygen in very small amounts whenthe bottom photoelectric conversion layer 5 and the front photoelectricconversion layer 8 are formed.

The concentration of carbon in the bottom photoelectric conversion layer5 can be made higher than the concentration of carbon in the frontphotoelectric conversion layer 8 by adding CH₄ (methane) in very smallamounts to SiH₄ (silane) serving as a raw material gas as well asadjusting the amount of the addition when the bottom photoelectricconversion layer 5 and the front photoelectric conversion layer 8 areformed, for example.

The concentration of nitrogen in the bottom photoelectric conversionlayer 5 can be made higher than the concentration of nitrogen in thefront photoelectric conversion layer 8 by adding NH₃ (ammonia), or bothNH₃ and H₂ in very small amounts to SiH₄ serving as a raw material gasas well as adjusting the amount of the addition when the bottomphotoelectric conversion layer 5 and the front photoelectric conversionlayer 8 are formed.

Furthermore, the concentration of oxygen in the bottom photoelectricconversion layer 5 can be made higher than the concentration of oxygenin the front photoelectric conversion layer 8 by adding CO₂ (carbondioxide) in very small amounts to SiH₄ serving as a raw material gas aswell as adjusting the amount of the addition when the bottomphotoelectric conversion layer 5 and the front photoelectric conversionlayer 8 are formed.

A method of manufacturing a stacked photovoltaic device according to thepresent embodiment will be then described. First, a supporting substrate100 is prepared, which comprises a polyimide resin film 2 having athickness of approximately 20 μm obtained by vapor depositionpolymerization on a stainless plate 1 such as SUS 430 having a thicknessof 0.15 mm, for example.

A back metal electrode 3 composed of Ag having a thickness ofapproximately 200 nm is then formed on the supporting substrate 100using an RF (high-frequency) magnetron sputtering method.

Thereafter, gases are successively introduced into a reaction chamber ina CVD system, to form a bottom cell 200 and a front cell 300 by a plasmaCVD method in the following manner. First, SiH₄, H₂, and PH₃ (phosphine)are introduced into the reaction chamber, to form an n-layer 4 having athickness of 20 nm on the back metal electrode 3. Then, SiH₄ and H₂ areintroduced into the reaction chamber, to form a bottom photoelectricconversion layer 5 having a thickness of 2 μm on the n-layer 4. Further,SiH₄, H₂, and B₂H₆ (diborane) are introduced into the reaction chamber,to form a p-layer 6 having a thickness of 20 nm on the bottomphotoelectric conversion layer 5. Thus, the bottom cell 200 is formed.

Thereafter, SiH₄, H₂, and PH₃ are introduced into the reaction chamber,to form an n-layer 7 having a thickness of 20 nm on the p-layer 6. Then,SiH₄ is introduced into the reaction chamber, to form a frontphotoelectric conversion layer 8 having a thickness of 300 nm on then-layer 7. Here, the concentration of carbon, the concentration ofnitrogen and the concentration of oxygen in the front cell 300 arecontrolled by adjusting reaction pressure, high-frequency power, and agas flow rate. Further, SiH₄, H₂, CH₄, and B₂H₆ are introduced into thereaction chamber, to form a p-layer 9 having a thickness of 20 nm on thefront photoelectric conversion layer 8. Thus, the front cell 300 isformed.

A transparent front electrode 10 composed of ITO having a thickness ofapproximately 80 nm is then formed on the p-layer using an RF magnetronsputtering method. Finally, a collection grid electrode 11 composed ofAg is formed on the transparent front electrode 10 by a vapordeposition.

In the stacked photovoltaic device according to the present embodiment,the concentration of impurities in the bottom photoelectric conversionlayer 5 is higher than the concentration of impurities in the frontphotoelectric conversion layer 8. Thus, light degradation of the frontcell 300 and light degradation of the bottom cell 200 by lightirradiation are balanced. As a result, deterioration of the outputcharacteristics of the whole stacked photovoltaic device is restrainedby long-term use, so that the low-term power generation is improved.

Another Modified Example

Although in the stacked photovoltaic device according to theabove-mentioned embodiment, the one bottom cell 200 is provided betweenthe supporting substrate 100 and the front cell 300, a plurality ofbottom cells may be stacked between the supporting substrate 100 and thefront cell 300. In the case, the same effect as that in theabove-mentioned embodiment is also obtained. However, a photoelectricconversion layer in the first photovoltaic cell from the side of a lightincidence surface is composed of amorphous silicon, and a photoelectricconversion layer in the second or subsequent photovoltaic cell iscomposed of microcrystalline silicon. The concentration of impurities inthe photoelectric conversion layer in the second or subsequentphotovoltaic cell must be higher than the concentration of impurities inthe photoelectric conversion layer in the first photovoltaic cell.

Although in the above-mentioned embodiment, the supporting substrate 100has a stacked structure of the stainless plate 1 and the polyimide resinlayer 2, the present invention is not limited to the same. For example,the stainless plate 1 may be replaced with other metal plates composedof iron, molybdenum, aluminum, etc. or various types of alloy plates.

Furthermore, although in the above-mentioned embodiment, the polyimideresin layer 2 is used as an insulating layer for electrically separatingthe photovoltaic cell from such a metal plate or alloy plate, thepresent invention is not limited to the same. For example, the polyimideresin layer 2 may be replaced with another resin layer composed of PES(polyether sulfone) or the like or an insulating thin film composed ofSiO₂ (silicon dioxide) or the like.

A combination of a material for the metal plate or the alloy platecomposing the supporting substrate 100 and a material for the insulatinglayer is not limited. For example, a combination of arbitrary materialscan be used.

Although in the above-mentioned embodiment, the surface of thesupporting substrate 100 is formed so as to be flat, the surface of thesupporting substrate 100 may have an irregular structure. For example,the surface of the supporting substrate 100 can be formed in anirregular shape by containing particles of SiO₂, TiO₂, etc. having adiameter of several micrometers in a resin layer such as the polyimideresin layer 2. In this case, light is scattered at the back of thestacked photovoltaic device, so that the effect of confining light isimproved. Thus, the conversion efficiency can be further improved.

Although in the above-mentioned embodiment, an n-type hydrogenatedmicrocrystalline silicon film is used as a semiconductor of oneconductivity type and a p-type hydrogenated microcrystalline siliconfilm is used as a semiconductor of the other conductivity type in thebottom cell 200, and an n-type hydrogenated microcrystalline siliconfilm is used as a semiconductor of one conductivity type and a p-typehydrogenated amorphous silicon carbide film is used as a semiconductorof the other conductivity type in the front cell 300, the presentinvention is not limited to the same. For example, a p-type hydrogenatedmicrocrystalline silicon film may be used as a semiconductor of oneconductivity type and an n-type hydrogenated microcrystalline siliconfilm may be used as a semiconductor of the other conductivity type inthe bottom cell 200, and a p-type hydrogenated microcrystalline siliconfilm may be used as a semiconductor of one conductivity type and ann-type hydrogenated amorphous silicon film may be used as asemiconductor of the other conductivity type in the front cell 300.

Furthermore, the crystalline properties of the other layers excludingthe bottom photoelectric conversion layer 5 are not limited in thebottom cell 200. The n-layer 4 and the p-layer 6 may be composed of amicrocrystalline silicon film, or may be composed of an amorphoussilicon film.

Similarly, the crystalline properties of the other layers excluding thefront photoelectric conversion layer 8 are not limited in the front cell300. The n-layer 7 and the p-layer 9 may be composed of amicrocrystalline silicon film, or may be composed of an amorphoussilicon film.

Although in the above-mentioned embodiment, (phosphorus) is used as ann-type dopant for the n-type layer 4 and the n-type layer 7, the presentinvention is not limited to the same. For example, a group V elementsuch as As (arsenic) may be used as an n-type dopant. Although in thepresent embodiment, B (boron) is used as a p-type dopant for the p-layer6 and the p-layer 9, the present invention is not limited to the same.For example, a group III element such as Al (aluminum) or Ga (gallium)may be used as a p-type dopant.

Examples

In inventive examples 1 to 3, described below, stacked photovoltaicdevices were formed by the method according to the above-mentionedembodiment, to measure the output characteristics and the impurityconcentration thereof. In comparative examples 1 to 3, stackedphotovoltaic devices were formed in the same method as that in theinventive examples except for the formation conditions of frontphotoelectric conversion layers 8, to measure the output characteristicsand the impurity concentration thereof.

(1) Inventive Example 1 and Comparative Example 1

Table 1 shows the formation conditions of the stacked photovoltaicdevice in the inventive example 1, and Table 2 shows the formationconditions of the stacked photovoltaic device in the comparative example1.

TABLE 1 Substrate High-frequency Inventive temperature Reaction pressurepower Gas flow rate example 1 [° C.] [Pa] [W] [sccm] Bottom n-layer 160133 100 SiH₄ 3 cell H₂ 200 PH₃ 0.6 Bottom 200 133 30 SiH₄ 20photoelectric H₂ 400 conversion layer p-layer 160 133 240 SiH₄ 10 H₂2000 B₂H₆ 0.2 Front n-layer 160 133 100 SiH₄ 3 cell H₂ 200 PH₃ 0.6 Front160 11 5 SiH₄ 30 photoelectric conversion layer p-layer 160 33 240 SiH₄10 H₂ 90 CH₄ 10 B₂H₆ 0.4

TABLE 2 Substrate Reaction High-frequency Comparative temperaturepressure power Gas flow rate example 1 [° C.] [Pa] [W] [sccm] Bottomn-layer 160 133 100 SiH₄ 3 cell H₂ 200 PH₃ 0.6 Bottom 200 133 30 SiH₄ 20photoelectric H₂ 400 conversion layer p-layer 160 133 240 SiH₄ 10 H₂2000 B₂H₆ 0.2 Front n-layer 160 133 100 SiH₄ 3 cell H₂ 200 PH₃ 0.6 Front160 133 30 SiH₄ 30 photoelectric H₂ 75 conversion layer p-layer 160 33240 SiH₄ 10 H₂ 90 CH₄ 10 B₂H₆ 0.4

As shown in Table 1 and Table 2, reaction pressure and high-frequencypower at the time of formation of the front photoelectric conversionlayer 8 in the inventive example 1 were made lower than those in thecomparative example 1. H₂ serving as a diluent gas was not introduced atthe time of formation of the front photoelectric conversion layer 8 inthe inventive example 1, and H₂ serving as a diluent gas was introducedat the time of formation of the front photoelectric conversion layer 8in the comparative example 1.

The respective initial characteristics of the stacked photovoltaicdevices in the inventive example 1 and the comparative example 1 weremeasured under conditions such as AM (Air Mass) of −1.5, 100 mW/cm², and25° C. Thereafter, the stacked photovoltaic device in each of theinventive example 1 and the comparative example 1 was divided into twoparts. One of the parts was used for evaluating characteristics afterlight irradiation, described later. In order to evaluate theconcentration of carbon as the concentration of impurities within eachof the bottom photoelectric conversion layer 5 and the frontphotoelectric conversion layer 8, the other part was analyzed by asecondary ion mass analyzer (SIMS; Secondary Ion Mass Spectroscopy).

First, Table 3 shows the results of the analysis by the SIMS. Theanalysis by the SIMS was carried out by using IMS-6F manufactured byCAMECA Instruments JAPAN KK and irradiating Cs⁺ ions at an angle ofincidence of 25 degrees at an acceleration voltage of 14.5 kV.

TABLE 3 Concentration of Concentration of impurities (carbon) inimpurities (carbon) in front photoelectric bottom photoelectricconversion layer conversion layer [atom/cm³] [atom/cm³] Inventive 6 ×10¹⁷ 4 × 10¹⁸ example 1 Comparative 7 × 10¹⁸ 4 × 10¹⁸ example 1

As shown in Table 3, the concentration of carbon in the bottomphotoelectric conversion layer 5 in the inventive example 1 isapproximately equal to that in the comparative example 1, while theconcentration of carbon in the front photoelectric conversion layer 8 inthe inventive example 1 is lower than that in the comparative example 1.Thus, the concentration of carbon in the bottom photoelectric conversionlayer 5 is higher than the concentration of carbon in the frontphotoelectric conversion layer 8 in the inventive example 1. On theother hand, the concentration of carbon in the bottom photoelectricconversion layer 5 is lower than the concentration of carbon in thefront photoelectric conversion layer 8 in the comparative example 1.

The results have shown that the concentration of carbon in the frontphotoelectric conversion layer 8 can be controlled by adjusting theformation conditions of the front photoelectric conversion layer 8.

Furthermore, in order to evaluate the conversion efficiency afterstabilization by light irradiation for a long time period, light wasirradiated for 160 minutes toward the respective other parts of thestacked photovoltaic devices in the inventive example 1 and thecomparative example 1 under conditions such as AM-1.5, 500 mW/cm², 25°C., and an opened state between terminals. Standardized outputcharacteristics were calculated by dividing the value of the outputcharacteristics after light irradiation by the value of the initialcharacteristics before light irradiation. Table 4 shows the standardizedconversion efficiency, standardized open-circuit voltage, standardizedshort-circuit current, and standardized fill factor (F. F.) as thestandardized output characteristics.

TABLE 4 Standardized Standardized Standardized conversion open-circuitshort-circuit Standardized efficiency voltage current fill factorInventive 0.88 0.99 0.97 0.92 example 1 Comparative 0.81 0.98 0.97 0.85example 1

The value of the standardized output characteristic is equal to thevalue of (1 light degradation factor) Consequently, the closer the valueof the standardized output characteristic is to one, the less the lightdegradation is.

As shown in Table 4, it is found that the light degradation factor inthe inventive example 1 is lower than that in the comparative example 1.The reason for this is considered as follows.

That is, the light degradation of the bottom photoelectric conversionlayer 5 can be brought close to the light degradation of the frontphotoelectric conversion layer 8 by making the concentration of carbonin the bottom photoelectric conversion layer 5 higher than theconcentration of carbon in the front photoelectric conversion layer 8,so that the light degradation of the bottom photoelectric conversionlayer 5 and the light degradation of the front photoelectric conversionlayer 8 are balanced. Therefore, the fill factor after light irradiationis kept high. Since the bottom photoelectric conversion layer 5 iscomposed of microcrystalline silicon, the light degradation thereof isinherently very little. In the stacked photovoltaic device, the bottomcell 200 and the front cell 300 are connected in series. In thecomparative example 1, therefore, the balance between the outputcharacteristics of the front cell 300 and the output characteristics ofthe bottom cell 200 is disrupted as the fill factor of the front cell300 is reduced. Therefore it is considered that the fill factor of thewhole stacked photovoltaic device was degraded. Contrary to this, in theinventive example 1, the output characteristics of the front cell 300and the output characteristics of the bottom cell 200 are balanced inthe inventive example 1, so that it is considered that the fill factorof the whole stacked photovoltaic device was also kept relatively high.

As a result of these, the output characteristics after the lightdegradation of the stacked photovoltaic device can be kept high bymaking the concentration of carbon in the bottom photoelectricconversion layer 5 higher than the concentration of carbon in the frontphotoelectric conversion layer 8.

(2) Inventive Example 2 and Comparative Example 2

Table 5 shows the formation conditions of the stacked photovoltaicdevice in the inventive example 2, and Table 6 shows the formationconditions of the stacked photovoltaic device in the comparative example2.

TABLE 5 Substrate High-frequency Inventive temperature Reaction pressurepower Gas flow rate example 2 [° C.] [Pa] [W] [sccm] Bottom n-layer 160133 100 SiH₄ 3 cell H₂ 200 PH₃ 0.6 Bottom 200 133 30 SiH₄ 20photoelectric H₂ 400 conversion layer p-layer 160 133 240 SiH₄ 10 H₂2000 B₂H₆ 0.2 Front n-layer 160 133 100 SiH₄ 3 cell H₂ 200 PH₃ 0.6 Front160 11 5 SiH₄ 30 photoelectric conversion layer p-layer 160 33 240 SiH₄10 H₂ 90 CH₄ 10 B₂H₆ 0.4

TABLE 6 Substrate Reaction High-frequency Comparative temperaturepressure power Gas flow rate example 2 [° C.] [Pa] [W] [sccm] Bottomn-layer 160 133 100 SiH₄ 3 cell H₂ 200 PH₃ 0.6 Bottom 200 133 30 SiH₄ 20photoelectric H₂ 400 conversion layer p-layer 160 133 240 SiH₄ 10 H₂2000 B₂H₆ 0.2 Front n-layer 160 133 100 SiH₄ 3 cell H₂ 200 PH₃ 0.6 Front160 266 30 SiH₄ 30 photoelectric H₂ 90 conversion layer p-layer 160 33240 SiH₄ 10 H₂ 90 CH₄ 10 B₂H₆ 0.4

As shown in Table 5 and Table 6, reaction pressure and high-frequencypower at the time of formation of the front photoelectric conversionlayer 8 in the inventive example 2 were made lower than those in thecomparative example 2. H₂ serving as a diluent gas was not introduced atthe time of formation of the front photoelectric conversion layer 8 inthe inventive example 2, and H₂ serving as a diluent gas was introducedat the time of formation of the front photoelectric conversion layer 8in the comparative example 2.

The respective initial characteristics of the stacked photovoltaicdevices in the inventive example 2 and the comparative example 2 weremeasured under conditions such as AM-1.5, 100 mW/cm², and 25° C.Thereafter, the stacked photovoltaic device in each of the inventiveexample 2 and the comparative example 2 was divided into two parts. Oneof the parts was used for evaluating characteristics after lightirradiation, described later. In order to evaluate the concentration ofnitrogen as the concentration of impurities in each of the bottomphotoelectric conversion layer 5 and the front photoelectric conversionlayer 8, the other part was analyzed by an SIMS.

First, Table 7 shows the results of the analysis by the SIMS.

TABLE 7 Concentration of Concentration of impurities (nitrogen)impurities (nitrogen) in front photoelectric in bottom photoelectricconversion layer conversion layer [atom/cm³] [atom/cm³] Inventive 6 ×10¹⁶ 1 × 10¹⁷ example 2 Comparative 2 × 10¹⁷ 1 × 10¹⁷ example 2

As shown in FIG. 7, the concentration of nitrogen in the bottomphotoelectric conversion layer 5 in the inventive example 2 isapproximately equal to that in the comparative example 2, while theconcentration of nitrogen in the front photoelectric conversion layer 8in the inventive example 2 is lower than that in the comparative example2. Thus, the concentration of nitrogen in the bottom photoelectricconversion layer 5 is higher than the concentration of nitrogen in thefront photoelectric conversion layer 8 in the inventive example 2. Onthe other hand, the concentration of nitrogen in the bottomphotoelectric conversion layer 5 is lower than the concentration ofnitrogen in the front photoelectric conversion layer 8 in thecomparative example 2.

The results have shown that the concentration of nitrogen in the frontphotoelectric conversion layer 8 can be controlled by adjusting theformation conditions of the front photoelectric conversion layer 8.

Furthermore, in order to evaluate conversion efficiency afterstabilization by light irradiation for a long time period, light wasirradiated for 160 minutes toward the respective other parts of thestacked photovoltaic devices in the inventive example 2 and thecomparative example 2 under conditions such as AM-1.5, 500 mW/cm², 25°C., and an opened state between terminals. Standardized outputcharacteristics were calculated by dividing the value of the outputcharacteristics after light irradiation by the value of the initialcharacteristics before light irradiation. Table 8 shows the standardizedconversion efficiency, standardized open-circuit voltage, standardizedshort-circuit current, and standardized fill factor as the standardizedoutput characteristics.

TABLE 8 Standardized Standardized Standardized conversion open-circuitshort-circuit Standardized efficiency voltage current fill factorInventive 0.88 0.99 0.98 0.91 example 2 Comparative 0.82 0.99 0.97 0.85example 2

As shown in Table 4, it is found that the light degradation factor inthe inventive example 2 is lower than that in the comparative example 2.The reason for this is considered as follows.

That is, the light degradation of the bottom photoelectric conversionlayer 5 can be brought close to the light degradation of the frontphotoelectric conversion layer 8 by making the concentration of nitrogenin the bottom photoelectric conversion layer 5 higher than theconcentration of nitrogen in the front photoelectric conversion layer 8,so that the light degradation of the bottom photoelectric conversionlayer 5 and the light degradation of the front photoelectric conversionlayer 8 are balanced. Therefore, the fill factor after light irradiationis kept high. Since the bottom photoelectric conversion layer 5 iscomposed of microcrystalline silicon, the light degradation thereof isinherently very little. In the stacked photovoltaic device, the bottomcell 200 and the front cell 300 are connected in series. In thecomparative example 2, therefore, the balance between the outputcharacteristics of the front cell 300 and the output characteristics ofthe bottom cell 200 is disrupted as the fill factor of the front cell300 is reduced. Therefore, it is considered that the fill factor of thewhole stacked photovoltaic device was degraded. Contrary to this, in theinventive example 2, the output characteristics of the front cell 300and the output characteristics of the bottom cell 200 are balanced inthe inventive example 1, so that it is considered that the fill factorof the whole stacked photovoltaic device was also kept relatively high.

As a result of these, the output characteristics after the lightdegradation of the stacked photovoltaic device can be kept high bymaking the concentration of nitrogen in the bottom photoelectricconversion layer 5 higher than the concentration of nitrogen in thefront photoelectric conversion layer 8.

(3) Inventive Example 3 and Comparative Example 3

Table 9 shows the formation conditions of the stacked photovoltaicdevice in the inventive example 3, and Table 10 shows the formationconditions of the stacked photovoltaic device in the comparative example3.

TABLE 9 Substrate High-frequency Inventive temperature Reaction pressurepower Gas flow rate example 3 [° C.] [Pa] [W] [sccm] Bottom n-layer 160133 100 SiH₄ 3 cell H₂ 200 PH₃ 0.6 Bottom 200 133 30 SiH₄ 20photoelectric H₂ 400 conversion layer p-layer 160 133 240 SiH₄ 10 H₂2000 B₂H₆ 0.2 Front n-layer 160 133 100 SiH₄ 3 cell H₂ 200 PH₃ 0.6 Front160 11 5 SiH₄ 30 photoelectric conversion layer p-layer 160 33 240 SiH₄10 H₂ 90 CH₄ 10 B₂H₆ 0.4

TABLE 10 Substrate Reaction High-frequency Comparative temperaturepressure power Gas flow rate example 3 [° C.] [Pa] [W] [sccm] Bottomn-layer 160 133 100 SiH₄ 3 cell H₂ 200 PH₃ 0.6 Bottom 200 133 30 SiH₄ 20photoelectric H₂ 400 conversion layer p-layer 160 133 240 SiH₄ 10 H₂2000 B₂H₆ 0.2 Front n-layer 160 133 100 SiH₄ 3 cell H₂ 200 PH₃ 0.6 Front160 400 30 SiH₄ 30 photoelectric H₂ 150 conversion layer p-layer 160 33240 SiH₄ 10 H₂ 90 CH₄ 10 B₂H₆ 0.4

As shown in Table 9 and Table 10, reaction pressure and high-frequencypower at the time of formation of the front photoelectric conversionlayer 8 in the inventive example 3 were made lower than those in thecomparative example 3. H₂ serving as a diluent gas was not introduced atthe time of formation of the front photoelectric conversion layer 8 inthe inventive example 3, while H₂ serving as a diluent gas wasintroduced at the time of formation of the front photoelectricconversion layer 8 in the comparative example 3.

The respective initial characteristics of the stacked photovoltaicdevices in the inventive example 3 and the comparative example 3 weremeasured under conditions such as AM-1.5, 100 mW/cm², and 25° C.Thereafter, the stacked photovoltaic device in each of the inventiveexample 3 and the comparative example 3 was divided into two parts. Oneof the parts was used for evaluating characteristics after lightirradiation, described later. In order to evaluate the concentration ofoxygen as the concentration of impurities in each of the bottomphotoelectric conversion layer 5 and the front photoelectric conversionlayer 8, the other part was analyzed by an SIMS.

First, Table 11 shows the results of the analysis by the SIMS.

TABLE 11 Concentration of Concentration of impurities (oxygen) inimpurities (oxygen) in front photoelectric bottom photoelectricconversion layer conversion layer [atom/cm³] [atom/cm³] Inventive 5 ×10¹⁸ 7 × 10¹⁸ example 3 Comparative 8 × 10⁸ 7 × 10¹⁸ example 3

As shown in Table 11, the concentration of oxygen in the bottomphotoelectric conversion layer 5 in the inventive example 3 isapproximately equal to that in the comparative example 3, while theconcentration of oxygen in the front photoelectric conversion layer 8 inthe inventive example 3 is lower than that in the comparative example 3.Thus, the concentration of oxygen in the bottom photoelectric conversionlayer 5 is higher than the concentration of oxygen in the frontphotoelectric conversion layer 8 in the inventive example 3. On theother hand, the concentration of oxygen in the bottom photoelectricconversion layer 5 is lower than the concentration of oxygen in thefront photoelectric conversion layer 8 in the comparative example 3.

The results have shown that the concentration of oxygen in the frontphotoelectric conversion layer 8 can be controlled by adjusting theformation conditions of the front photoelectric conversion layer 8.

Furthermore, in order to evaluate the conversion efficiency afterstabilization by light irradiation for along time period, light wasirradiated for 160 minutes toward the respective other parts of thestacked photovoltaic devices in the inventive example 1 and thecomparative example 3 under conditions such as AM-1.5, 500 mW/cm², 25°C., and an opened state between terminals. Standardized outputcharacteristics were calculated by dividing the value of the outputcharacteristics after light irradiation by the value of the initialcharacteristics before light irradiation. Table 12 shows thestandardized conversion efficiency, standardized open-circuit voltage,standardized short-circuit current, and standardized fill factor as thestandardized output characteristics.

TABLE 12 Standardized Standardized Standardized conversion open-circuitshort-circuit Standardized efficiency voltage current fill factorInventive 0.87 0.98 0.98 0.91 example 3 Comparative 0.81 0.99 0.98 0.84example 3

As shown in Table 4, it is found that the light degradation factor inthe inventive example 3 is lower than that in the comparative example 3.The reason for this is considered as follows.

That is, the light degradation of the bottom photoelectric conversionlayer 5 can be brought close to the light degradation of the frontphotoelectric conversion layer 8 by making the concentration of oxygenin the bottom photoelectric conversion layer 5 higher than theconcentration of oxygen in the front photoelectric conversion layer 8,so that the light degradation of the bottom photoelectric conversionlayer 5 and the light degradation of the front photoelectric conversionlayer 8 are balanced. Therefore, the fill factor (F.F.) after lightirradiation is kept high. Since the bottom photoelectric conversionlayer 5 is composed of microcrystalline silicon, the light degradationthereof is inherently very little. In the stacked photovoltaic device,the bottom cell 200 and the front cell 300 are connected in series. Inthe comparative example 3, therefore, the balance between the outputcharacteristics of the front cell 300 and the output characteristics ofthe bottom cell 200 is disrupted as the fill factor of the front cell300 is reduced. Therefore, it is considered that the fill factor of thewhole stacked photovoltaic device was degraded. Contrary to this, in theinventive example 3, the output characteristics of the front cell 300and the output characteristics of the bottom cell 200 are balanced, sothat it is considered that the fill factor of the whole stackedphotovoltaic device was also kept relatively high.

As a result of these, the output characteristics after the lightdegradation of the stacked photovoltaic device can be kept high bymaking the concentration of oxygen in the bottom photoelectricconversion layer 5 higher than the concentration of oxygen in the frontphotoelectric conversion layer 8.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A stacked photovoltaic device having a light incidence surface,comprising a plurality of photovoltaic cells stacked and each includinga photoelectric conversion layer composed of a substantially intrinsicsemiconductor, the photoelectric conversion layer in the onephotovoltaic cell closest to the light incidence surface including anamorphous semiconductor, and the photoelectric conversion layer inanother photovoltaic cell including a non-single crystallinesemiconductor containing crystal grains, and the concentration ofimpurities contained in the photoelectric conversion layer in said otherphotovoltaic cell being higher than the concentration of impuritiescontained in the photoelectric conversion layer in said one photovoltaiccell.
 2. The photovoltaic device according to claim 1, wherein saidnon-single crystalline semiconductor is a microcrystalline semiconductorcontaining crystal grains having a diameter of not more than 1 μm. 3.The photovoltaic device according to claim 1, wherein said impuritiesinclude carbon, the concentration of carbon contained in thephotoelectric conversion layer in said other photovoltaic cell beinghigher than the concentration of carbon contained in the photoelectricconversion layer in said one photovoltaic cell.
 4. The photovoltaicdevice according to claim 1, wherein said impurities include nitrogen,the concentration of nitrogen contained in the photoelectric conversionlayer in said other photovoltaic cell being higher than theconcentration of nitrogen contained in the photoelectric conversionlayer in said one photovoltaic cell.
 5. The photovoltaic deviceaccording to claim 1, wherein said impurities include oxygen, theconcentration of oxygen contained in the photoelectric conversion layerin said other photovoltaic cell being higher than the concentration ofoxygen contained in the photoelectric conversion layer in said onephotovoltaic cell.
 6. A method of manufacturing a stacked photovoltaicdevice, comprising the steps of: forming a plurality of photovoltaiccells in order, each comprising a photoelectric conversion layercomposed of a substantially intrinsic semiconductor, the photoelectricconversion layer in the one photovoltaic cell closest to a lightincidence surface including an amorphous semiconductor, and thephotoelectric conversion layer in another photovoltaic cell including anon-single crystalline semiconductor containing crystal grains; andadjusting at least one of the formation condition of the photoelectricconversion layer in said one photovoltaic cell and the formationcondition of the photoelectric conversion layer in said otherphotovoltaic cell such that the concentration of impurities contained inthe photoelectric conversion layer in each of said other photovoltaiccell is higher than the concentration of impurities contained in thephotoelectric conversion layer in said one photovoltaic cell.